It is proposed to investigate the nature of changes in speed and efficiency of cognitive functioning which may occur across the lifespan of the normal human being. Birren (1974) has suggested the aged person undergoes a generalized slowing down of the central nervous system, in which all processes involved in normal mental function are progressively impaired. This notion is difficult to assess since there is the clear possibility of confounding peripheral deterioration with true central impairment. The present proposal will attempt to pinpoint the source of response slowing with age by the use of a technique which allows not only the measurement of response time but also inference of the speed of mental transformations. Four highly competent adult age groups (18-29, 31-49, 50-64, 65 and above) will be asked to perform the mental image rotation task described by Cooper and Shepard (1973). Birren's central slowing hypothesis predicts not only increased response time but also decreased speed of mental manipulation, indicative of a slowing of internal processes. A second critical variable (in addition to age) is stimulus familiarity. It is predicted that elderly people will have relatively more difficulty in processing unfamiliar information than will younger subjects. Individual subject differences will be analyzed in order to determine whether or not differing processing styles may be characteristic of the various age groups.